I've been a fan of science fiction and fantasy for many years, and I can't really imagine something else that captured my attention so completely. Now, looking back, I find it funny and somewhat disturbing that any story which contained a trace of the supernatural had my undivided interest.
I still enjoy fantasy, but not as whole-heartedly as before. In fact, I haven't thought about the genre for a while, until I picked up a collection of fairy tale retellings by Robin McKinley. I'd admired her characterizations of brave and headstrong heroines in The Hero and the Crown and The Blue Sword, so I expected to at least enjoy the characters, even if the plot was a given.
What I found most intriguing, however, wasn't the characters: it was the world in which each story was set. For example, one of the stories depended on the defeat of a wizard by water from a pool which was immune to magic. Being fairly slow on the uptake, it took me a while to figure out how the jug of water saved the day. Only after I remembered the example of a charmed necklace being neutralized after a dip in the pool did I understand the point the plot hinged on.
But I can't entirely blame myself, because I wasn't entirely sure how magic worked in the world of the story. I interpreted the scene with the necklace as simply demonstrating that enchantments could be removed by water, but I never imagined the evil wizard as being constituted of enchantments.
The point of the above rant is this: is world-building essential in all science fiction and fantasy? Having been once confused by insufficient details in a story, I would like to say, yes, it is important to have a well defined world in which magic obeys its own laws ... but I'm hesitant to apply it to all science fiction and fantasy. There may be certain instances in which it is better to leave the story a little mysterious and frightening. Having made this exception, I would prefer fantasy and science fiction with a world easily understandable. For example, Harry Potter would never have achieved its popularity if J.K. Rowling hadn't specified the limits of the world: the dead can't be revived, food can't be conjured out of thin air, certain spells and curses are illegal. Having made all this clear, Rowling's readers wouldn't find reading as frustrating. Instead of feeling their way around an imaginary world, they live comfortably in it, enjoying life as it might be.